A web material can undergo specific transformations when passed between a first and second roll. The first and second roll may consist of a tool roll and an anvil roll. For instance, the web material passing between the first and second roll may undergo an embossment, a cutting into separate sheets, or a coating of a layer of material, etc.
The gap between the first and second roll is critical in order to perform the desired transformation on the web material. If the gap between the first and second roll is too high in comparison to a target distance (a target gap between the first and second roll), the product or the transformation quality can be adversely affected. If the gap between the first and second roll is too low in comparison to the target distance, the first and second roll contact each other which can lead to an excessive wear of the first and second roll or even blocking of the first and second rolls.
In the industry, proper execution of an entire process may often rely upon an operator. The operator initially sets up the distance between the first and second roll such that the target distance is met. The operator sets up the initial positions of the first and second roll. During the continuous process, the operator may have to make some adjustments to reposition the first and/or second roll such that the gap between the first and second roll continuous to meet the target distance. Without knowing precisely the actual gap, the operator's actions are based on the evaluation of the product quality and “trial-and-error”.
However, such adjustments can be time-consuming and require stopping the process. Also these adjustments may not be accurate, which may lead to either an unsatisfactory web transformation or to premature equipment wear.
Also, when installing the first and second roll, the operator may have to take into account the expansion of the first and second roll during the start-up due to heat generated by the apparatus itself or near-by equipment. The heat may be due to the rotation of the first and second roll. The gap between the first and second roll decreases due to the expansion in diameter of the first and second roll. As a consequence, the first and second roll may come into contact with each other, which leads to unsatisfactory transformation of the web material and an increased wear of the rolls.
There is thus a desire to develop an automated method in order to adjust the gap between the first and second roll which has been initially set-up by the operator. There is a desire that the gap between the first and second roll continuously meets the target distance and does not require the involvement of the operator.
A web cutting machine having temperature measuring and control means in order to maintain the selected cutting gap between the first and second roll has been disclosed in U.S. Pat. No. 3,186,275.
There is thus a desire to develop a method and apparatus to infer the gap between the first and second roll. The measuring device to be selected should be suited for use in a plant. Hence, the method and apparatus have to be as universal as possible to be applicable to as many types of web transformation as possible. The cost may be as low as possible. Any measuring device may have a lifetime as long as possible and hence in order also to withstand conditions in a manufacturing plant, there is a desire to select a measuring device according to its robustness against dust and debris.
Web material transformation by passing between the first and second roll is typically a continuous process. There is thus a desire to develop a method suitable for a continuous process to ongoingly correct the gap to a target distance (a target gap) in order to ensure a satisfactory web transformation without the need to stop the continuous process.
Also, there is a desire to prevent any equipment wear in order to continuously ensure a satisfactory web transformation.